1. Field of the Invention
The present invention relates to the fields of antennas for space applications. The present invention specifically relates to a printed quasi-tapered tape helical element and quasi-tapered helical array antenna.
2. Description of the Prior Art
Traditionally, helix antennas are realized with electrical conductors such as solid or hollow conductors, supported by an electrically insulating material. Sometimes, a helix conductor is extended through a balun to the coaxial connector as an electrical connection. The dielectric arms extending from the metallic support at the centre are used to support the helix antennas. These helix antennas are designed to radiate in an axial mode of operation, i.e. maximum radiation along the axis of the helix antenna, where the basic design equations are well established in the literature for initial design. The end tapering reduces the reflected wave, i.e. standing wave distribution of current over helix turns from the open end of the helix antenna.
U.S. Pat. No. 4,169,267 and U.S. Pat. No. 5,345,248 describe the helical antenna that generates a radiation beam with reduced on axis axial ratio. In U.S. Pat. No. 4,169,267, two different tapers are utilized for optimizing both gain and axial ratio as shown in FIG. 1, which illustrates a helix element geometry configured with various combinations of tapered diameter and uniform sections 100 and 102. Such a helical antenna utilizes two uniform sections 100 and 102 of helix and two tapered sections 101 and 103 of the helix for different frequency bands. However, this conventional approach neither improves off axis axial ratio nor gives compact size of helix 4 wavelengths).
Moreover, U.S. Pat. No. 5,258,771 describes interleaving of the array of different frequency band elements to achieve the dual band performance of the helical array. However, this exhibits an inherent limitation of loading high frequency helix array by the low frequency helix array. Hence, the performance of high frequency helix array is compromised while comparing with low frequency helix array. In an antenna array configuration, each radiator is placed at the nodes of an array lattice to ensure effective radiation efficiency. Each radiator in the presence of an array of radiators suffers from mutual coupling, which degrades the axial ratio and the main beam symmetry. The distance between each radiator cannot be increased as it is dictated by the gain of individual element and requirement of high gain of array antenna.
U.S. Pat. No. 5,345,248 describes a scheme of reducing the mutual coupling effect by staggering the radiators in an amount equal to one turn of a helix length along the axial direction. However, this concept cannot be used in the direct radiating feed array configuration. Further, the feeding mechanism also becomes most critical which controls both impedance matching and insertion loss. Thus, it is more desirable to have direct mounting of helix to the feeding transmission line. U.S. Pat. No. 6,816,126 B2 describes a scheme of feeding the tape helical element and circular helical array with parallel plate feeding mechanism, but such feeding mechanism cannot be extended to other transmission line. Also, the conventional helix antenna impedance matching is accomplished by using additional conducting strip loading at helix feeding point or balun circuit.
In general, the circular polarization purity in terms of off-axis axial ratio performance over the wideband is essential for wide beam space borne antennas, i.e. navigation, mobile and communication satellite antennas. Nominally, the off-axis axial ratio less than 3 dB is the acceptable performance but the advanced satellite technology requires axial ratio less than 2 dB over the global coverage. With respect to the conventional approaches, the uniform helix antenna with conventional support for the helix conductor achieves inherent high axial ratio (on-axis and off-axis) performance. This is mainly due to the current distribution over the helix conductor, which generates standing wave patterns. The low axial ratio (on-axis and off-axis) performance over the wide band is difficult to achieve as compared to the return loss performance when helix axial length is less than one wavelength. In such a conventional helix antenna, the on-axis axial ratio performance improves up to the certain value with an increase in the number of turns while the off-axis axial ratio remains high. Therefore, it is desirable to provide a printed quasi-tapered tape helical array antenna, which achieves low axial ratio (on-axis and off-axis) performance over the wideband for global coverage.